Why T. Rex's Puny Arms Are a Sign of Strength

And other strange facts about dinosaurs that will help us prepare for the future.

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A life-size model of the Cretaceous predator Spinosaurus illustrates some of the amazing adaptations that made dinosaurs so dominant for so long.

Photograph by Mike Hettwer, National Geographic Creative

PUBLISHED September 30, 2017

When an asteroid slammed into Earth 65 million years ago, the resulting giant tsunamis, earthquakes, and catastrophic global warming wiped out the planet’s dominant species—dinosaurs. There were probably more than 10,000 different species, many of them still unknown to us, says Kenneth Lacovara in his new book, Why Dinosaurs Matter.

As a paleontologist and director of the Edelman Fossil Park at Rowan University in New Jersey, Lacovara hates it when people use the word “dinosaur” to describe something as out of date. When he spoke to National Geographic, he explained why he also objects to jokes about T. rex’s puny arms and why dinosaurs can help us navigate our uncertain path into the future.

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Courtesy of Simon and Schuster

You loathe the term “dinosaur” when applied to things that are outdated or obsolescent. Explain your frustration.

I do! [Laughs] Dinosaurs were globally dominant creatures on land for the better part of 165 million years. They’re cosmopolitan in every type of environment and every ecological niche. During this entire time, our ancestors are these little shoelace creatures, hiding in the dark and forgotten recesses of the dinosaur world, hoping not to be noticed.

Then a cosmic accident occurs and the dinosaurs get snuffed out by a space rock that unleashed hell on Earth. Yet some of them survived in the form of birds, which today outnumber mammal species by three to one. So dinosaurs are still with us, still remarkably persistent and adaptive.

No one tarnishes the legacy of Albert Einstein because he eventually died. Nobody diminishes the accomplishments of Neil Armstrong because after he walked on the moon, he grew old and died. So why would we use that same argument to tarnish the legacy of dinosaurs? When I hear people say, “This company or political party is a dinosaur,” I think, they should hope to be so lucky!

The most iconic dinosaur, beloved of Hollywood and children’s toys, is the T. rex. What made it so exceptional?

The best-known dinosaurs today, like T. rex, Stegosaurus, and Brontosaurus, were among the very first dinosaurs described and the first to enter the popular consciousness, so they have been bouncing around our culture longer than other dinosaurs.

When T. rex was described in the early part of the last century, it was the biggest meat eater that had ever been seen, and it’s still the biggest. It’s huge, with this ferocious bite. Paleontologists who have looked at the biomechanics of the T. rex jaw believe it has the most powerful bite force of any animal on land.

There are all these memes making fun of its puny little arms—the joke is that T. rex can’t put on its hat, clap its hands or wipe its bum. But a colleague of mine, Michael Habib in Los Angeles, has looked closely at this.

He thinks that to have a strong bite, you need big, powerful jaw muscles. To have these, you need a big head, so the muscles can attach. If you have a big head, you also need big neck muscles to support that head. But you can’t have a big head and big arms—neck and arm muscles compete for muscle attachment space across the bones of the shoulder. The puny little arms people make fun of are one of the keys to the power and ferocity of this amazing animal.

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The tiny arms of Tyrannosaurus Rex made its ferocious bite possible.

Photograph by Richard Nowitz, National Geographic Creative

Most of us can probably only name one or two other dinosaur species—but there was immense variety, wasn’t there?

Pun intended, we have only begun to scratch the surface of dinosaur biodiversity. Paleontologists know of over 1,000 to 1,500 species of dinosaurs. But there were probably many more—tens of thousands over their entire reign. They’re this amazingly rich, biodiverse group of organisms.

If you go back 100 years ago, the rate of discovery was about one new dinosaur species per year. As you get to the 1970s, when there are more paleontologists and jet travel becomes possible, the rate goes up to about half a dozen a year. Last year there were 36 new dinosaur species described. The rate of discovery is amping up every year with no sign of it ending. We have a long way to go before we hit “peak dinosaur.”

You use the term “deep time” to describe the planet’s past. Explain what you mean by that—and why it took so long for the fossil and geological record to be accepted.

Our human lives play out over such a short period of time, just a handful of decades, which makes it hard for us to conceive of even 1,000 years. To try to conceive of millions or tens of millions—and the Earth is four and a half billion years old!—is almost impossible. People think of dinosaurs as being really old, but when the non-bird dinosaurs went extinct, 66 million years ago, that was only 2 percent of Earth’s history ago. If you take all of human history—from the agricultural revolution to the industrial revolution and, today, the space age—geologically that’s all, basically, now. Draw out the time scale on a sheet of paper and all of Earth’s history will fit in the breadth of your last pencil stroke.

The literal, geological interpretation of the Bible began with James Ussher, a bishop and primate of Ireland, who wanted to find out how old the Earth was. The best resource as far as he was concerned was the Bible. He determined the age of King Nebuchadnezzar and from there went back through all the “begats” in the Bible to the time of Adam and Eve. He determined that the Earth was created in 4004 B.C., specifically on October 23rd at about 6pm on a Saturday. [Laughs]

Of course, it’s demonstrably incorrect. We have trees alive today that are older than 6,000 years old. If you’re alive today and you think the Earth is only 6,000 years old, that means you reject evolution, which means you reject all of biology, physics, chemistry, astronomy, and geology. It is truly a medieval point of view. It doesn’t make any sense.

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This nodosaur fossil discovered by miners in Canada is the best preserved fossil of its kind ever found. Read about the discovery.

Photograph by Robert Clark, National Geographic Creative

You describe in detail the apocalypse when an asteroid crashed into the earth. Talk us through that event—and explain why dinosaurs disappeared but other forms of life survived that eventually led to us?

The asteroid, about 10 kilometers across, is going roughly 45,000 mph or about 25 times the speed of a bullet. Compared to the Earth it’s rather tiny. But the force released is not just according to the size; it’s the mass times the velocity. It hits the Earth and blows a crater in the ground that’s about the size of Massachusetts and 20 miles deep.

Sean Gulick, a geophysicist at the University of Texas, says that the argument right now in his geophysics community is whether that event heated up the atmosphere to the temperature of a toaster oven for a few hours or a pizza oven for a few minutes. Either way, your dinosaur is dead.

All kinds of other terrible things happened as well. A magnitude-10 earthquake radiated across the Earth at 17,000 mph; giant tsunami waves raked across the coastlines. And all the dust and rock, forest fires and ash that went up into the atmosphere shrouded the sun.

In the ocean, the food chain is really short, only a few weeks from primary productivity of the phytoplankton to apex predators like tuna fish. Once you have plankton breakdown, pretty soon there’s nothing to eat in the ocean. If you are big and consume a lot, you die. If you’re little, like small fish, or eat infrequently, like crocodiles and some turtles, you might make it through. But big things in the ocean, like mosasaurs, which weren’t actually dinosaurs, all go extinct.

On land, the largest animal that survives is about the size of a cat. All of the dinosaurs that were here for so long perished except for one group, the birds, though only some of the birds make it through. Why is that? Every non-avian dinosaur egg nest that has ever been found has been in a crater in the ground, which some mama dinosaur scraped out and then laid her eggs in. One hundred percent of their life cycle is on the surface of the Earth. But today we have examples of burrowing birds, like parrots that nest in the banks of the Amazon or burrowing desert owls. The one group of dinosaurs that we know for sure has some burrowing members is the one that gets through the extinction.

Bring it home for us, Ken. Why do dinosaurs matter?

Dinosaurs matter because the future matters and everybody, including paleontologists, is concerned more with the future than the past. But we don’t have access to the future. Nobody remembers the future, you can’t do experiments in the future, and the present is gone before you can think of it. So, the only information we have to help us plot our course into the future is from the past. As Winston Churchill said, “The further back you look, the further ahead you will see.”

If you look at the ancient record, there have been catastrophes, climate change, sea level rise, and biodiversity crises, accompanied by unbelievable adaptations and problem solving in biomechanics. So, there are answers in the ancient record and we would be foolish and arrogant to ignore it. The book is called “Why Dinosaurs Matter,” but it’s really about why the past matters. And the past matters because the future matters.